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Description of Event During the summer months, 011 of Colorado is subject to convective thunder- storms. When sufficient humidity is present in the atmosphere, some of these storms produce large amounts of rainfall in short time periods. The source of the humidity is typically moisture from the Gulf of Mexico and the central plains states which, under certain weather patterns, can drift into eastern Colorado. Also, in mid and late summer, a monsoon wind circulation brings moisture, origi. noting from either or both the Baja California area and/or the Gulf of Mexico, northward up across Mexico into the Southwest. The moisture source is respon- sible for many of the summer thunderstorms over Colorado's mountains and western volleys. In late July 1997, tropical moisture was streaming northward across Mexico and the Southwest into Colorado. At the same time, 0 large high-pressure sys- tem stalled over the central high plains of the United States. The system's clock- wise rotation pumped very humid airfrom the central plains and Gulfof Mexico into eastern Colorado. A cold front, associated with the high pressure area over the northern plains provided a trigger to set offthunderstorms as moist airmasses converged over Colorado. In the 3-4 days preceding, July 28, 1997, the City of Fort Collins and most of eastern Colorado received soaking and/or drenching rains, adding to soil mois- ture in some locations. As the cold front arrived in the late afternoon of July 27th, strong thunderstorms developed just north and west of Fort Collins. Later that night, steady rains developed along the eastern base of the foothills in Larimer County and continued until about noon, July 28th. Several inches of new rain were reported iust west and northwest of Fort Collins totally saturating the ground, producing major flooding in Laporte, and setting the stage for the evening flood event. On the evening of July 28, 1997, intense rains began around 6:30 p.m. in the foothills west of Fort Collins. Winds from the east and southeast continued to pump moisture into the storm system throughout the evening. The core of the storm was very small but remained nearly stationary over the headwaters of Spring Creek, the Fairbrooke Channel, Clearview Channel, the CSU Drainage Bosin and the West Vine Drainage Basin. Rainfall intensity increased and reached a maximum between 8:30 p.m. and 10:00 p.m. before ending abruptly. A subsequent analysis of rainfall conducted by CSU showed a maximum of 10.2 inches of rainfall in less thon five hours near the intersection of Drake Rd. and Overland Trail.